1. Field of the Invention
The present invention is directed to an electrostatic thermal bonding of glass substrates, and in particular to an improved electrostatic thermal bonding method capable of easily and solidly bonding a pair of glass substrates without using an adhesive or a high-temperature process.
2. Description of the Background Art
An electrostatic thermal bonding method was introduced in 1969, and then a silicon-glass bonding and a silicon-silicon bonding utilizing a glass thin film have been used in packaging of miniature devices. The thermal expansion coefficient of a glass used for the electrostatic thermal bonding is almost identical with that of a silicon, and materials containing metallic elements therein having a remarkable ionization tendency such as natrium and lithium are used for the glass to be bonded. For the silicon-glass bonding, when the silicon substrate and glass substrates are brought face to face with each other and heated to a predetermined temperature, and then the metal elements inside the glass substrates are ionized into the ionic condition having constant electric charges. Here, in the case that an external direct current voltage of a cathode is applied to the glass substrates, the ionized metal ions are moved into the cathode due to the electric field generated at both end portions of the silicon-glass interface and a space charge section is formed at the interface of the silicon-glass. A strong electrostatic force is generated due to the space charges. The silicon and the glass substrates are forced to be pulled toward each other by the electrostatic force, and thus a bonding of the silicon and the glass substrates is performed.
However, in the case of a flat-plate display device, the bonding of the two glass substrates is conventionally performed utilizing a method for bonding a pair of glass substrates by putting glass frit on a surface of one glass substrate, facing the pair of substrates to one another, and heating the glass substrates to a temperature of about 450 to 600.degree. C. for melting the glass frit, whereby the pair of glass substrates are bonded
As mentioned above, the conventional electrostatic thermal bonding methods have been limited to a silicon-glass bonding and silicon-silicon bonding utilizing a glass thin film. That is, the conventional electrostatic thermal bonding methods have not been adopted for a glass--glass bonding related to a display device.
In the studies recently performed with regard to the packaging of flat-plate display devices, a glass--glass bonding using a glass frit is utilizing for bonding glass substrates. However, such a packaging, i.e., the conventional glass--glass bonding method has disadvantages in that the longevity of the device is reduced due to the high temperature bonding and the device is contaminated by gas generated during the melting of the glass frit, which results in inferior effect on the condition that a vacuum packaging should be performed.